[ticket/12951] Added editorconfig

https://tracker.phpbb.com/browse/PHPBB3-12951
There are literally no advantages to using spaces for indentation.

Tabs for indentation, spaces for alignment. [Lea Verou agrees](http://lea.verou.me/2012/01/why-tabs-are-clearly-superior/).

Seriously though, [nobody cares](http://seldo.tumblr.com/post/78622304911/tabs-vs-spaces).